The subject matter of the present invention is a method for the preparation of alkyl orthoesters of titanium, zirconium or hafnium by the esterification of the tetrahalides with aliphatic alcohols in two steps.
The known methods for the preparation of orthoesters, especially of the metals titanium, zirconium and hafnium, avail themselves exclusively, even on a large technical scale, of the reaction of the corresponding tetrachlorides with aliphatic alcohols in the presence of amines as acid acceptors and solvents as diluents (cf. U.S. Pat. No. 2,187,821). This method yields usable products, yet it involves some serious disadvantages. A very special disadvantage is the great and unavoidable production of unsalable amine hydrochlorides. Moreover, the titanium and zirconium orthoesters thus prepared contain, in addition to the solvent and alcohol residues usually still present, considerable amounts of polymers. These form on the basis of the formation of alkyl halide, occuring as a secondary reaction, in which water is simultaneously formed, which has a hydrolyzing effect on the metal ester along with a simultaneous, quality-degrading formation of metal oxane.
In order at least to lessen the unavoidable production of salts, it is proposed in British patent 997,892 to esterify the titanium tetrachloride directly with the evolution of gaseous hydrogen halide, and then using a vacuum to remove the hydrogen halide. This method of direct esterification with aliphatic alcohols, however, does not result in the orthoester, but only reaches the dihalogen diester (semiester) and then stops, even when there is an excess of alcohol. The introduction of the last two alkoxy groups in the preparation of the orthoesters must, in this procedure, take place in a second step in the presence of amines. In this procedure the disadvantage of the known methods--the tendency toward the secondary reaction that forms alkyl halide and water and thus the formation of polymers in the esterification products--is not eliminated. And this problem was not been resolved, too, by the application of solvents and/or boiling the reaction mixture after the introduction of the compounds and/or complicating the process by bubbling through inert gases as proposed in British Pat. No. 987,892. Therefore these since 1963 created methods are not in large scale application up today. Especially titanium esters and zirconium esters are useful in many ways on account of their properties, and have a permanent place in a number of applications. For example, they serve as catalysts and co-catalysts for the polymerization and copolymerization of olefins, vinyl chloride, styrene, dienes, vinyl ethers, epoxides, alkylene oxides, and aldehydes, as esterification and transesterification catalysts in organic, organic silicon and organic metal monomer chemistry, and in the preparation of saturated and unsaturated polyesters, polyester amides and imides, and polyamides by means, for example, of the condensation and polyaddition reactions; in lacquers and resins as binding agent, for thixotropication, for the modification of rheological properties, and as fabricating adjuvants; for the surface treatment of glass and mineral substances; as processing agents for textiles, leather and paper, for example, especially for binding, repellentizing or delustering; for the manufacture of special ceramics, as adhesive components in adhesives, and in the manufacture of glass fiber-reinforced plastics, etc.
The problem therefore arises of finding a more efficient method of preparation, in which the above-described secondary reactions occur to a minor extent or not at all, so that orthoesters are obtained very free of polymers, and in which there is only a markedly reduced creation, if any, of ammonium halides or amine hydrochlorides.